Imaging electron energy filters, also known as electron filters, energy filters, or electron energy spectrometers are used in transmission electron microscopes to improve the contrast of the image of the specimen by selecting electrons of a particular energy range. This is most clearly apparent with non-contrasting, very thin specimens having a selected energy loss (.DELTA.E) in the range of from approximately 100 to 200 eV. However, even an image of a specimen produced with purely elastically scattered electrons (energy loss .DELTA.E=0), which is done by filtering out any non-elastically scattered electrons (.DELTA.E&gt;0), is markedly improved in contrast as compared with the unfiltered image.
A further substantial advantage of a transmission electron microscope having an imaging electron filter is that element-specific images (that is, element distribution images) can be made simultaneously over a relatively wide specimen range in that the energy range allowed to pass through the electron filter corresponds to an element-specific interaction of the transmitted electrons with the specimen, or in other words corresponds to a K, L or M absorption in the shell of the atom. Thus, it is possible to obtain qualitative distribution images, and quantitative distribution images as well if the intensity ratios are measured and the background is subtracted, of the elements in thin specimens (.ltoreq.30nm) that have very high local resolution (.congruent.0.5 nm) and maximum detection sensitivity (.congruent.2.10.sup.-21 g); until now, this was unattainable by any other analysis technology. Maximum local resolution and maximum sensitivity of detection of elements are highly important both in biological and medical research and the study of materials.
In electron diffraction diagrams, as well, an imaging electron filter produces sharper images of the diffraction diagram by filtering out non-elastically scattered electrons. It is also possible to make diffraction diagrams of non-elastically scattered electrons in a particular energy range.
German Pat. No. 20 28 357 discloses an electron microscope which enables filtering of the image of the specimen or of the image of the diffraction diagram. An electron filter that comprises a magnetic prism and an electrostatic mirror is used. The electrostatic mirror, however, is highly sensitive to external stray fields; moreover, at high diffraction voltages for the electrons, problems of insulation also arise. For this reason, purely magnetic filters have been preferred of late. These energy filters are classified as so-called alpha and omega filters in accordance with the course of the electron paths.
Omega filters are known from various publications. In this connection, reference may be made to an article in Optik, Volume 43, page 495 (1975) by G. Zanchi et al wherein an apparatus is described which includes three magnets and three deflection regions, with deflection angles of 116.degree., 232.degree. and 116.degree.. Several publications for apparatus having four deflection regions and deflection angles of 90.degree. or less are also known, for example: H. Rose et al, Optik, Volume 40, page 336 (1974); D. Krahl et al, Ninth International Congress on Electron Microscopy, Toronto, Volume 1, page 42 (1978); and, H. Wollnik et al, Optik, Volume 46, page 255 (1976).
To be optimally usable in an electron microscope, an imaging electron filter must meet the following two conditions: first, a large number of image points must be transmitted without loss of local resolution; and second, the imaging errors in the exit source plane (also called a selection plane) must be so slight that narrow energy widths (on the part of the transmitted electrons) can be realized.
In the electron filters known from the above publications, the attainable energy widths are unsatisfactory in each case; in many cases, the local resolution is inadequate as well.
From a publication by H. Rose et al in Optik, Volume 54, page 235 (1979), an apparatus is disclosed which likewise includes four deflection regions with deflection angles of 90.degree.. By providing the pole pieces with curved edges and by providing three additional hexapoles, a virtually complete correction of the imaging errors is obtained. However, this apparatus has the disadvantage of high production cost, especially for the curved edges. Also, this electron filter is difficult to adjust because of the curved edges.